Intrinsically-calibrated tribometer

ABSTRACT

Intrinsically calibrating friction mensuration device has a drive unit with controllable motive member, configured to receive a controllable propulsion force, and to apply tractive force to a test piece on a surface. A force engine couples to the motive member, and applies the controllable propulsion force. The device includes a controller imposing a propulsion control signal actuating the force engine to produce the propulsion force. A force sensor is between the drive unit and the test piece, producing a force signal responsive to a sensed force between the drive unit and the test piece. The drive unit can self-propelled or stationary. A method includes applying an increasing tractive force to a test piece at rest; measuring the tractive force; identifying the maximum tractive force applied at incipient motion; identifying the transition between the resting test piece and incipient motion; determining the SCOF; continuing motion; and determining the DCOF.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of test measurementand more particularly to friction mensuration devices and mostparticularly to tribometers.

2. Background Art

Over one million people experience slip, trip, or fall accidents eachyear and approximately 17,000 die from the resultant fall injuries,second only to the number of automobile accident fatalities. Theseaccidents are particularly serious for the elderly, who may suffer afall injury that severely impairs their mobility, independence, andgeneral health. These accidents can and do occur in a wide range ofenvironments, including construction sites, private residences,playgrounds and recreational facilities, industrial settings, public andprivate property (e.g., sidewalks, highways/roadways, streets, parkinglots, markets); public transportation facilities (airports, subways,train & bus stations) and facilities for the elderly and disabled.Hence, a great deal of attention is placed on the quality of walkwaysurfaces used for ambulation by pedestrians.

The criteria for walkway surface safety are determined by consensus of apanel of safety experts, based on measurements of the coefficient ofstatic friction (SCOF) of surfaces under scrutiny. In general,physicists define the SCOF as the ratio of the maximum tangential forceneeded to cause a standard surface to slip divided by the force holdingthe surfaces in contact, herein called the normal force. A value of 0.5for SCOF has been adopted as a threshold of safety for a walkwaysurface.

It is believed that the most representative measurements of a surfaceare taken in the field, that is, at the location of the surface to betested. A large number of friction-measuring field devices (herein,tribometers) are used to perform friction measurements. Each type offield device employs a different effector mechanism for tribologicmensuration. Examples of tribometers include a variable incidencetribometer (VIT), a variable angle tribometer (VAT), a portableinclinable articulated strut slip tester (PIAST), and a family ofarticulated strut tribometers known in the art as Brunbager Mk. Islipmeter, Brunbager Mk. II slipmeter, and Brunbager Mk. III slipmeter,named after the device developer. An archetypal impact device is the“James” machine, is a bench mounted device, suitable for the laboratoryand not the field. Each of the aforementioned tribometers may becharacterized as an “impact”-type device, in which the parametermeasured results from an impact between a test surface and a materialsample affixed to the particular tribometer. Similar devices, such asthe Sigler Pendulum or the British Pendulum, employ principles of theConservation of Momentum to measure the kinetic forces corresponding topendulum motion being arrested by a test material.

Each type of impact device is operator dependent, that is, the criteriafor the critical onset of motion during testing cannot be uniquelydetermined. In addition, the “impact” aspect of these devices impliesthat a kinetic phenomenon is being used to determine a staticphenomenon, namely the static coefficient of friction. Application ofthe First Law of Thermodynamics to the mechanics of impact devices showsthat these devices dissipate some amount of kinetic energy duringmensuration, causing wide variability in the measured value of “static”friction. Thus, a measurement reported by an impact-type device is anestimated proxy measurement for the static frictional force and, byextension, for SCOF. Not unsurprisingly, impact tribometers tend togenerate non-reproducible, divergent numerical values for the samesurface and condition under test. In fact, “impact devices” that rely ongravity, on spring loaded pressure, or on pneumatic pressure areactually energy converters that tend not to reliably produce theirtheoretically expected horizontal and vertical force components. Inaddition to the impact-type tribometer, a “pull”-type tribometer also isin use. Two devices exemplary of this type are known to ones of ordinaryskill in the art as a horizontal pull slipmeter, and a horizontaldynamometer pull-meter. Each device relies on an operator applying ahorizontal force on the slipmeter test device, which is coupled to thetest sample. The point of motion initiation being taken as correspondingto the SCOF. Some “slipmeters” are closer in physical characteristics tospring-loaded fishing or laboratory scales. Again, an operator applies ahorizontal force and notes the point motion begins. Variations inducedby the operator are inevitable. The subjective and objective variabilityof device operation, unintentional or unavoidable application ofnon-horizontal force, and vagaries in result interpretation, can lead toa myriad of results for the same test conditions. Moreover, each of the“pull”-type devices use a respective test material (neolite,neoprene/rubber) that is generally unrepresentative of materials used byan actual pedestrian. A difficulty in reported and publishedmeasurements derived from impact and pull devices alike is that thedevices produce divergent numerical values for their output. In 2005,testimony at an OSHA hearing revealed there was no test procedure orinstrument that accurately correlates to the actual human walking andworking condition. Thereafter, ASTM International (formerly, AmericanSociety for Testing and Materials) withdrew International Certificationfor all field measurement protocols for Coefficient of Friction, with noreplacements. Hence, there is a need for a tribometer that provides adirect measurement for static friction force, which isoperator-independent, and which produces results that are reliable,reproducible, and verifiable.

SUMMARY

Embodiments herein provide friction mensuration apparatus and methodsfor friction mensuration. The friction mensuration device includes adrive unit having at least one controllable motive member coupled to thedrive unit, configured to receive a controllable propulsion force, andconfigured to apply tractive force to a test piece on a surfaceresponsive to the controllable propulsion force. The device alsoincludes a force engine coupled to the at least one controllable motivemember, and configured to apply the controllable propulsion force to theat least one controllable motive member. In addition, the deviceincludes a controller coupled to the force engine, and imposing apropulsion control signal to the force engine, in which the propulsioncontrol signal is actuating the force engine to produce the controllablepropulsion force. Furthermore, the device includes a force sensorcoupled between the drive unit and the test piece, the force sensorproducing a force signal responsive to a sensed force between the driveunit and the test piece. In the device, the drive unit is intrinsicallycalibrated. The friction mensuration device of claim 1, wherein thedrive unit is a self-propelled drive unit coupled to the test piece. Insome embodiments the drive unit is a stationary drive unit coupled tothe test piece. In other embodiments the drive unit is a self-propelleddrive unit coupled to the test piece.

An embodiment of the apparatus includes the test piece having apreselected test material thereon, wherein the test piece is oriented toplace the preselected test material in contact with a test surface, andwherein the test piece receives the tractive force. The sample carriertranslates the tractive force into a test force applied to thepreselected material sample, and the test force applied to thepreselected material sample is representative of a friction forcebetween the preselected material sample and the surface. The sensedforce can be a static friction force, or a dynamic friction force.

In another embodiment, the device, in response to a received forcesignal, the controller actuates the force engine to produce acontrollable propulsion force on the at least one controllable motivemember and, responsive thereto, imposes a friction force imposed betweenthe test surface and the preselected sample material. In yet anotherembodiment, the friction mensuration device further includes amensuration memory coupled to the controller and having memory storageunits. The mensuration memory is configured to receive from thecontroller a plurality of representations of the sense force, and toretain respective ones of the plurality of representations of the senseforce. In still other embodiments, the test piece is a contact sledfurther including a carrier having facets. Each of the facets includes asample carrier thereon, with each sample carrier being disposed toreleasably retain a respective preselected material sample. The carriercan be disposed to position one of the preselected test materials incontact with the surface.

In other embodiments of the friction mensuration device, the forceengine applies the tractive force during a first portion of a testepoch, corresponding to the static coefficient of friction. In stillanother embodiment of the device, one of the preselected samplematerials is a control sample material. In yet other embodiments, thecontrol sample material is a piece of footwear or is a resin rubberconstituent of a preselected rubber soling material.

An embodiment of a method of friction mensuration, includes applying anincreasing tractive force to a test piece at rest on a test surface atthe beginning of a test epoch; measuring the tractive force on the testpiece at rest on a test surface during the test epoch; identifying themaximum tractive force applied at incipient motion of the test piece onthe test surface during the test epoch; identifying a nonce of the testepoch signifying the transition between the test piece at rest andincipient motion; determining the static coefficient of friction of thetest piece relative to the test surface for the test epoch; andmeasuring the tractive force used to continue motion in the test sledfor the test epoch. The test period can be created from a plurality ofsequential test epochs. In a method embodiment of friction mensurationthe test piece is stopped between selected test epochs in the testperiod.

In other embodiments of the method above can be included applyingtractive force to the test piece during a calibration period; suspendingthe test piece separated from the test surface during the calibrationperiod; and weighing the suspended test piece, wherein the weighing is aconstituent of intrinsically calibrating a friction mensuration device.In yet other embodiments, the method further includes continuing thetractive force during the test epoch; stopping the tractive force at theend of a test epoch; determining the static coefficient of friction ofthe test piece relative to the test surface; and determining the dynamiccoefficient of friction of the test piece relative to the test surface.The method can be for a intrinsically calibrated device, which methodincludes applying tractive force to the test piece during a calibrationperiod; suspending the test piece separated from the test surface duringthe calibration period; and weighing the suspended test piece, whereinthe weighing is a constituent of intrinsically calibrating a frictionmensuration device.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures herein provide illustrations of various features andembodiments in which:

FIG. 1A is an illustration of a model of tractive, frictional,gravitational, and normal forces used to effect the embodiments of thepresent invention;

FIG. 1B is an illustration of static and dynamic forces arising from themodel in FIG. 1A;

FIG. 2 is a side view illustration of one embodiment of anself-propelled intrinsically calibrated tribometer, in accordance withthe teachings of the present invention;

FIG. 3 is a block diagram model of an embodiment of an intrinsicallycalibrated tribometer, in accordance with the teachings of the presentinvention;

FIG. 4 is an exploded perspective view of an embodiment of anintrinsically calibrated tribometer in FIG. 2, in accordance with theteachings of the present invention;

FIG. 5 is a rear view of another embodiment of an intrinsicallycalibrated tribometer, in accordance with the teachings of the presentinvention;

FIG. 6 is a top view of the embodiment of an intrinsically calibratedtribometer shown in FIG. 5, in accordance with the teachings of thepresent invention;

FIG. 7 is a side view illustration of one embodiment of a stationaryintrinsically calibrated tribometer, in accordance with the teachings ofthe present invention;

FIG. 8 is an illustration of a multiple epoch test output; and

FIG. 9 is a side view illustration of another embodiment of a stationaryintrinsically calibrated tribometer, in accordance with the teachings ofthe present invention;

The embodiments of the invention and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments and examples that are described and/orillustrated in the accompanying drawings and detailed in the followingdescription. It should be noted that the features illustrated in thedrawings are not necessarily drawn to scale, and features of oneembodiment may be employed with other embodiments as the skilled artisanwould recognize, even if not explicitly stated.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B are physical and frictional models, respectively, whichillustrate the principles of the embodiments described herein. FIG. 1Adepicts a physical model 100 having two elements 105, 110 in frictionalcontact. In general, model 100 can be illustrated with a surface offirst element 105 being disposed second element 110. First element 105may be a test material. Second element 110 may be a test surface. Secondsurface 110 is depicted as being substantially horizontal, relative togravitational force G (115). On a terrestrial surface (e.g., an x, yplane in an x, y, z space), first element 105 experiences normal force N120 opposing gravitational force G 115. In general, normal force N 120is produced at a right angle)(

90° to the mutual interface between first element 105 and second element110, that is, where the opposing surfaces of first element 105 andsecond element 110 meet. When tangential force F 125, which is depictedas substantially horizontal, is gradually applied to first element 105,relative to second element 110, element 105 tends to remain at rest.Force F 125 is resisted by opposing frictional force f 130. As force F125 increases to a maximum value, relative motion will occur betweenfirst element 105 and second element 110. That is, first element 105will begin to slide over second element 110. The maximum value offrictional force f 130, at the point of movement, corresponds to a valuefor the coefficient of static friction (SCOF). Typically, a decreasedtangential force F 125 may be used to sustain the relative movement,once the critical value of measured force F is reached.

FIG. 1B illustrates a friction model 200, which can be representedwithin the context of physical model 100 in FIG. 1A, as measured bytangential force F 125, applied as a function of time t 205. Frictionforce can be manifest in two forms: (1) static friction, f_(S) 210 and(2) kinetic friction, f_(K) 220. In general, the laws of friction areempirical in character and approximate in their predictions. Two basicempirical laws of friction can be expressed as: (1) the static frictionis approximately independent of the area of contact over wide limits;and (2) the static frictional force is proportional to the normal forceN (e.g., force N 120). Typically, static friction force f_(S) 210 can beassociated with the force F 125 applied to at-rest first element 105 incontact with second element 110. As illustrated in FIG. 1B, the periodduring which this force F 125 is applied, and frictional force f resistsin the form of static friction force f_(S) 210, extends from the initialapplication of force F 125 at time t₀ 230 to the time at which appliedforce F 125 coincides with the initiation of motion in first element 105at time t_(M) 240. At time nonce t_(M) 240, contact is broken betweenopposing surfaces of first element 105 and second element 110, allowingfirst element 105 to move relative to second element 110. In thiscontext, such movement is called “sliding.” This point of maximum valueestablishes the value of f_(M) 225, whose magnitude defines one of thecomponents for determining the SCOF. Once sliding occurs, a smaller,kinetic, force, generally indicated by f_(K) 220, is needed to maintainmotion of the first element 105 in contact with second element 110. Theperiod during which this force F 125 is applied, and frictional force fresists, in the form of kinetic friction force f_(K) 220 extends fromthe initial movement of first element 105 at time t_(M) 240 until theforce F 125 is no longer measured, represented by time t_(END) 245.Often, dynamic forces and surface characteristics can provide greater orlesser resistance, as illustrated by oscillations 190 in f_(K) 220.

The empirical relationship between the magnitudes of the normalcomponent N 120 and the static frictional force f_(M) 225 at time t_(M)240, may be characterized by:

f _(M)=μ_(S) ×N  (EQ. 1)

where f_(M) 225 corresponds to the value for static friction force f_(S)210 at the moment of motion, t_(M) 240, μ_(S) corresponds to thecoefficient of static friction (SCOF), and N 120 corresponds to thenormal force at the mutual interface of elements 105 and 110. Asunderstood by those of ordinary skill in the art, an accurate andreliable measurement of f_(M) 225 can directly produce a SCOF withsimilar characteristics, whereas friction measurements derived fromdynamic processes may deliver unreliable, irreproducible, and divergentresults.

In embodiments of the present invention, force F 125 can be graduallyapplied in a uniformly or monotonically, increasing manner, thetransition between f_(S) 210 and f_(K) 220 can be readily detected,allowing direct measurement of maximum static force f_(M) 225. An eventduring which frictive force is measured is hereinafter designated as a“test,” although parameters other than friction force may be measuredduring a test.

For the purposes of the present description, the period of time betweenthe test initiation, t₀, 230 and test completion, t_(END), 245, willhereinafter be designated a “test epoch,” or simply, an “epoch.”Embodiments in accordance with the present invention can be configuredto perform tests and to collect information, including frictional forcedata, during one or more test epochs.

FIG. 2 illustrates a side view of an embodiment of a self-propelledembodiment of an intrinsically calibrated tribometer generally at 200,which can include functional units described in FIG. 3, with respect totribometer 300. Tribometer 200 is depicted in an example configuration,to illustrate features of certain self-propelled embodiments describedherein. Tribometer 200 can include drive unit 205, contact sled 215, andforce sensor 210 coupled between drive unit 205 and contact sled 215.Tribometer may be self propelled by at least one controllable motivemember 220 attached to the frame of drive unit 205, and configured toreceive a controllable propulsion force F_(T) 222 from force engine 225(not shown but typically within a case of drive unit 205). Force F_(T)causes tribometer 200 to be propelled in a selected direction (e.g., Xdirection) on test surface 250. Drive unit 205 may include one or moreguide rails 202, which may include one or more followers 208. Followers208 generally support contact sled 215 and assist rails 202 conveytractive force to sled 215. Guide rails 202 can be configured toslidingly support contact sled 215 relative to drive unit 205. Wheels288 typically do not come into contact with test surface 250 duringoperation but may be used to easily move tribometer 200 in a mannersimilar to a wheeled luggage carrier.

Embodiments of tribometer 200, relative to FIG. 1A, are configured tocontribute minimally, if at all, to the normal reactive force N, whichacts in opposition to the gravitationally-induced weight force W.Contact sled 215 can be disposed with slotted support frame 208, whichmay slidingly engage guide rails 202.

Intrinsically calibrated tribometer 200 can include a weight caisson 260in contact sled 215. Weights, such as sand- or lead-filled stockinettematerial can be disposed in caisson 260. Also caisson may be used inconjunction with stanchion 275 which may be used to receive calibratedweights, e.g., weight 290, to create a preselected sled weight forcaisson 260 and, by extension, sled 215. Sled 215 can include a facetedsample carrier 263 between caisson 260 and test surface 250. Each facet265, 275, of carrier 263 can be configured to retain thereon arespective predetermined test sample 267, 277 for frictional testingwith test surface 250. In FIG. 2, test sample 277 is depicted to be inflat, parallel contact with test surface 250.

Turning to FIG. 3, a block diagram illustrating an embodiment of thepresent invention is shown. Intrinsically calibrated tribometer 300 caninclude drive unit 305, force sensor 310 and contact sled 315.Tribometer 300 can be disposed on preselected test surface 350. Incertain embodiments, drive unit 305 is provided for self-propulsion oftribometer 300. Self-propulsion can be achieved by coupling drive unit305 to at least one controllable motive member 320, which can beconfigured to propel drive unit 305 on test surface 350. For example,controllable motive member 320 may be, without limitation, a wheel, atread, a ball, a roller, a drum, a coil, or a belt. Controllable motivemember 320 also may be an element configured to translate propulsiveforce into horizontal movement of tribometer 300, including, withoutlimitation, a leg/pedal-type element, a crawler-type element, aperistaltic-type element, or a helical-type element.

Controllable motive member 320 can be integral with drive unit 305, ormay be articulated, for example, by way of an axle, a spool, a spindle,a joint, a pendant, or a gearbox. At least one controllable motivemember 320 is configured to convert a received propulsive force intomotion. It is desirable that at least one controllable motive member 320be configured to propel drive unit 305, and by extension, intrinsicallycalibrated tribometer 300, smoothly to minimize irregularities in forcestransmitted mechanically through tribometer 300, and to providesubstantially uniform propulsion.

Drive unit 305 can include force engine 325 coupled to the at least onecontrollable motive member 320. Force engine 325 is configured to applya controllable propulsion force to at least one controllable motivemember 320; causing tribometer 300 to be propelled in a selecteddirection on test surface 302. Force engine 325 may incorporate asteering unit configured to directionally manipulate the at least onecontrollable motive member 320, causing drive unit 305 to change thedirection of propulsion. Force engine 325 may be, without limitation, arotating electric motor, which may be operated by 12V DC power. Anexample of such a motor may be a 12VDC REVERSIBLE GEARED MOTOR, such asModel CRA203 (P/N 30570) from Cramer Co., South Windsor, Conn. USA. Ofcourse, force engine 325 may be another suitable motor such as, withoutlimitation, be a stepper motor, a linear DC motor, a brushed DC motor, abrushless DC motor, a piezoelectric motor, or other 5VDC to 24 VDC motorconfigured to produce linear self-propulsion of tribometer 300 frompower unit 390, whether directly or in conjunction with mechanicalmotion translation elements, which may be disposed in one or both offorce engine 325 or controllable motive member 320. Force engine 325 maybe coupled to force sensor 310 by a screw transmission, to permitmovement by infinitesimal, and nearly infinite, measures, allowing foran exceptional degree of precision currently unknown to the tribometerart.

Drive unit 305 also can include controller 330 coupled to force engine325 and configured to impose control signal 337 on force engine 325.Control signal 337 actuates force engine 325 to produce the controllablepropulsion force, F 125 from FIGS. 1-2. Typically, force engine 325produces the controllable propulsion force F 125 in a gradual, uniformlyincreasing manner 210, as illustrated in FIG. 2, responsive to controlsignal 337 from controller 330. Controller 330 captures a high-fidelityvalue of peak friction force f_(M) 225 at nonce t_(M) 240, that is, whencontact sled 315 begins to move, relative to test surface 350. Peakfriction force value f_(M) 225 can be used to determine SCOF (see EQ.1). A frictional force signal 333 received from force sensor 310 can besampled, for example, at a rate of about 5 samples per second to about100 samples per second for a period of time during which peak frictionforce value f_(M) 225 occurs. Sample values of frictional force signal333 can be stored in a memory, such as memory 355.

Controller 330 can be a microprocessor, a microcontroller, or a statemachine. An example of a suitable controller 330 includes, withoutlimitation, a low-power, CMOS AT90USB64 8-bit microcontrollermanufactured by Atmel Corporation, San Jose, Calif. USA. Controller 330may be an integrated controller, or may be representative of adistributed cooperative of interoperating control elements disposed indrive unit 305. Controller 330 can perform a “Power On Self-Test” (POST)to verify that elements are disposed in a proper power state (ON/OFF),that tribometer 300 is configured to perform a preselected tribometrictest in accordance with a predetermined tribometric process, and thatsensing and mensuration elements appear operational. In addition,controller 330 can communicate “INITIATE” and “STANDBY” commands tosensing and mensuration elements to place the elements in a knowninitial state, and to make ready tribometer 300 for a test procedure.

Memory 355 may be coupled to controller 330, or may be integratedtherein. Memory 355 may be used to store command, control, andcommunication information and code, which may facilitate semi-autonomousoperation of tribometer 300. Memory 355 also may be used to collect datafrom force sensor 310 during one or more testing periods. An example ofsuitable memory 355 can be 2 GB×8 NAND Flash Memory model MT29F2G08manufactured by Micron Technology, Inc., Boise, Id., USA. In addition,memory unit 355 also may be representative of one of the ubiquitousremovable memory devices (e.g., “stick,” “card,” “pen drive”), typicallyusing non-volatile FLASH memory technology, on which command, control,and sensed information may be at least temporarily stored. Alternately,memory unit 355 can be configured to transfer stored information to sucha removable memory device in the usual course of operations, forexample, autonomously, or upon command for such a transfer by basestation 311.

Controller 330 can be configured to actuate intrinsically calibratedtribometer 300 to perform one test during a test epoch t₀ to t_(M) (230,245), or to perform multiple tests during respective test epochs. Atesting period may contain one or more test epochs. Actuation oftribometer 300 may be performed by remote communication with controller330, for example, from base station 302. Intrinsically calibratedtribometer 300 also can be actuated by controller 330 in response to anapplied signal or environmental condition or by a signal from navigationunit 345. Controller 330 can store values corresponding to preselectedtest parameters, for example, in a memory within controller 330, inmemory 355, which is coupled within drive unit 305 to controller 330, orto a discrete memory device, such as a USB memory “stick” which can bephysically coupled to drive unit 305 and electrically coupled tocontroller 330 by way of communication unit 385.

Furthermore, controller 330 can be configured with well-known routinesto determine for each test epoch or test period, a maximum measuredfriction force, a corresponding SCOF for a test epoch or test period, acorresponding DCOF for an epoch or a test period, a time, a date,location indicia, an acceleration in one or more dimensions, an ambienttemperature, an ambient humidity, a pitch, a yaw, a forward or a lateralacceleration or a characterization of a physical characteristic.Acceleration, pitch, yaw, attitude, and absolute or relative positioninformation (represented as location indicia) may be obtained bycontroller 330 from navigation unit 345. Also, controller 330 may beconfigured to calculate statistical values corresponding toaforementioned data collected from selected testing epochs including,without limitation, calculating, an average value of f_(M), an averagevalue of SCOF, average deviation, standard deviation, probable error,average error, force skewness, or kurtosis, as well as test surfacetemperature, planarity, tilt, and surface and finish characteristics.Such data and the information that results therefrom can confirmaccuracy, reproducibility, and reliability of tribometer 300—obtaineddata over multiple tests, and can assist in analyzing the preselectedsample material, analyzing the preselected test surface, or both, or incharacterizing variability in data which may arise under particularcircumstances. Controller 330 also may be configured to record date andtime for each test data set, relative or absolute location or elevation,or measurement count.

In some selected embodiments, drive unit 305 can be supplied with acommunication unit 340. Communication unit 340 can be configured with atleast one of a wired communication port, a wireless communication port,a memory device port, or a display port. Incoming signals tocommunication unit 340 may include, without limitation, commands,identifiers, and data. Outbound signals from communication unit 340 mayinclude, without limitation, a force measurement value, a sensedenvironmental condition value, computed values, tribometer position,velocity, or acceleration, position, energy store level, or mesh unitsynchronization.

A wired communication port can be, without limitation, a UniversalSerial Bus (USB) port, IEEE 1394 serial bus port, an RS-232 serial busport, an RS-422 serial bus port, or an unshielded twisted pair (UTP)serial port having an RJ-11 connector. Communication unit 340 may haveone or more than one of such communication ports, which can be coupledto controller 330. The Atmel AT90USB64 8-bit microcontrollerincorporates, for example, Universal Serial Bus (USB) On-the-Gofunctionality to provide bidirectional wired communication betweentribometer 300 and a base station 302. Wireless communication betweentribometer 300 and base station 302 may be implemented in tribometer300, base station 302, or both, for example, using an USB adapter (orUSB dongle), coupled to, or integrated in communication unit 385. In onenon-limiting alternate example, a DWA-210 Xtreme N® Duo USB Adapter(dongle) available from D-Link Systems, Inc., Fountain Valley, Calif.,USA may be used. The DWA-210 wireless dongle provides wireless IEEE802.11a/b/g/n WLAN capability via a bidirectional USB link. Alternately,and without limitation, a WPAN wireless link may be used. For example,DBT-120 Wireless BLUETOOTH® 2.0 USB Adapter (dongle), also from D-LinkSystems, Inc., may implement IEEE 802.15.2/BLUETOOTH® functionality.Also, a WPAN implementation may use an @ANY2400 IEEE 802.15.4/ZigBee™USB Dongle, from Adaptable Network Solutions GmbH, Dresden, Del. Thewell-known IEEE 802.15.4 standards, or the ZigBee™ specifications, mayexemplify techniques for supporting a point-to-point bidirectional link,between base 302 and tribometer 300. However, an IEEE 802.15.4 standard,or the ZigBee™ specifications, also may provide a technique by which toimplement a cooperative or mesh network or other distributed,intelligent interaction between or among multiple ones 303, 304 oftribometer 300. The aforementioned dongle providers, and their vendors,also make available tutorial matter and software, by which such donglesmay be controlled. At least a portion of the software provided for arespective dongle may be programmed into memory unit 355. In yet anotheralternative embodiment, communication unit 385 may be communicativelycoupled to base station 302 by a wireless transceiver different from aUSB connection. For example, communication unit 385 may be configuredwith an electromagnetic communication transceiver, using electromagneticradiation in the established RF signal ranges, or in the establishedinfrared signal ranges. Acoustic or ultrasonic communication may beeffected by communication unit 385. One of ordinary skill in the artcould substitute other configurations and processors to provide similarfunctionality, in light of the teachings herein.

Controller 330 also may be coupled to navigation unit 345 disposed intribometer 300. Navigation unit 345 may be used to determine a locationof tribometer 300, geospatially, relative to base station 302, or both.In the present context, “determine” can mean to cause tribometer 300 tobe propelled to a preselected location, within a predetermined frame ofspatial reference (add XYZ to FIG. 3); to resolve the location oftribometer 300, within the predetermined frame of spatial reference; orboth. Navigation unit 345 can include, for example, an accelerometer, aninertial navigation unit, a global positioning system (GPS) unit, or acombination of an accelerometer and an inertial navigation unit, or anaccelerometer and a global positioning system (GPS) unit, anaccelerometer and an inertial navigation unit, or an inertial navigationunit and a global positioning system (GPS) unit. An accelerometer may bea 1-axis accelerometer, a 2-axis accelerometer, or a 3-axisaccelerometer. By increasing the number of axes along which measurementswere taken, an accelerometer in the context of the present invention,navigation unit 345 can provide increasing functionality includingsensing one or more of acceleration (translational) data, tilt data,shock data or vibration data. An inertial measurement unit (IMU) may bea 4-degrees-of-freedom (DoF), or a 6-DoF inertial measurement unit. Byincreasing the degrees of freedom in an inertial navigational unit inthe context of the present invention, navigation unit 345 can provideincreasing functionality including one or more of sensing accelerationdata, vibration data, rotation data, tilt data, shock data, and multipledegrees-of-freedom motion data. Such data may facilitate a pre-testcalibration of tribometer 300, an in-test or a post-test correction of aposition, a force, or a friction measurement by tribometer 300, orin-test correction of propulsion direction. Navigation unit 345, ifdisposed in the multiple ones 300, 303, or 304, can facilitatepurposeful coordinated mensuration of larger areas, for example,simultaneously. Navigation units may be configured with a cooperativeinertial navigation module, or a GPS navigation module, or a customnavigation module, to facilitate precise placement and movement of oneor more of intrinsically calibrated tribometers 300, 303, 304.Navigation may be initiated or controlled by base station 302, or it maybe initiated or controlled locally, for example, on tribometer 300, andreported to base station 302.

Non-limiting examples of an accelerometer can be an ADXL-103accelerometer (1-axis), an ADXL-203 or an ADIS-210003 accelerometer(2-axis), or an ADXL-327 accelerometer (3-axis), each from AnalogDevices, Inc., Norwood, Mass. USA. Non-limiting examples of an inertialmeasurement unit can be an ADIS-16300 (4-DoF) or an ADIS-16360 IMU(6-DOF), with each IMU also being produced by Analog Devices, Inc. Anon-limiting example of a GPS unit can be an STA2058 TESEO™ GPS platformIC, manufactured by STMicroelectronics, Geneva, CH. Of course, anartisan of ordinary skill could substitute other motion detectionelements without departing from the scope of the invention herein.

Contact sled 315 can be coupled to drive unit 305 so that contact sled315 receives at least a portion of the controllable propulsion force,and is drawn in the direction of drive unit 305. Contact sled 315 can beconfigured to have a test sample carrier 360 disposed thereon, which isconfigured to releasably retain preselected material sample 365.Preselected material sample 365 can be a material to be tested for slipor wear resistance, slipperiness, or other physical characteristic, whenapplied frictively to a test surface (not shown). Contact sled 315 canbe formed with a predetermined mass which, under gravitationalacceleration, will impart to contact sled 315 a predetermined weight.However, some types of friction mensuration may call for contact sled315 to have a greater mass or weight. To this end, some embodiments oftribometer 300 include weight caisson 360, which can be a shapedframework or container in which one or more slugs, each having apreselected slug weight, may be disposed in weight caisson to alter,typically increase, the predetermined weight of contact sled 315.Alternatively, weight caisson 360 may be disposed in drive unit 305, orin both contact sled 315 and drive unit 305, in accordance with abalanced weight distribution of tribometer 300. In general, contact sled315 orients sample carrier 363 to place preselected test material incarrier 365 in contact with the surface 350. Contact sled 315 conveysthe propulsion force to sample carrier 360 which, in turn, translatesthe at least a portion of the propulsion force into a test force appliedto the preselected material sample 365. Upon initiation of motion ofpreselected material sample 365, relative to test surface (not shown),the test force applied to the preselected material sample isrepresentative of a friction force between the preselected materialsample and the test surface. The test force corresponds to F 125 in FIG.1, and the friction force corresponds to force f_(M) 225, in FIG. 2.

In selected embodiments of tribometer 300, contact sled 315 may bedisposed with a carrier having facets 365, 375, with each facet 365, 375being disposed to releasably retain a respective preselected materialsample thereon. A multifaceted carrier 363 facilitates seriatimtribometry of multiple material samples. For example, first preselectedmaterial sample in carrier facet 365 may be used to determine a firstSCOF of a dry, clean preselected test surface and, thereafter, secondpreselected material sample in carrier facet 375 may be used todetermine a second SCOF after the preselected test surface has beenwetted, contaminated, or both. In other embodiments of tribometer 300,contact sled 315, itself, may be configured with facets. In onenon-limiting example, tribometer 300 may employ a contact sled 315,having a hexagonally-shaped cross-section and sixlongitudinally-oriented facet surfaces, with each disposed with arespective sample carrier 365, 375. As with other embodiments, eachrespective sample carrier 365, 375 can be disposed to releasably retaina respective preselected material sample 367, 377.

Force sensor 310 couples drive unit 305 to contact sled 315. As driveunit 305 is propelled, tension is applied to force sensor 310, in thecourse of transmitting the propulsion force to contact sled 315. Forcesensor 310 can be configured to sense a tensional force which maydevelop between drive unit 305 and contact sled 315, which tensionalforce is representative of at least a portion of the propulsion force.Force sensor 310 can be, without limitation, a load cell, or a strainsensor, configured to measure loads of between about 0.05 lbf to about20 lbf and typically, between about 0.1 lbf to about 10 lbf. One exampleof a load cell or strain sensor can be a 20-lb rated thin beam loadcell, P/N TBS-20 made by Transducer Techniques, Inc., Temecula, Calif.,USA, although other load cells or strain sensors may be used. Forcesensor 310 can generate force signal 333, and impose force signal 333 oncontroller 330. In general, force signal 333 can be representative of ahorizontal friction force created by an interface between test surfaceand a test sample in reaction to gravitational forces, W 115, a normalforce, such as N 120, and propulsive force F 125 in FIG. 1. Typically,force signal 333, when generated by force sensor 310 and imposed oncontroller 330, can cause controller 330 to actuate force engine 325 tocontrollably apply a motive force to controllable motive unit 335.

Base station 302 can be communicatively coupled with tribometer 300, forexample, by a wired link, a wireless link, or both. Base station 302 maybe used to transmit commands to, and to receive data from tribometer300. Base station 302 may be a handheld system or smaller, or it may bea laptop computer, a desktop computer, or an industrial controllerconfigured to communicate with at least one tribometer. In somein-the-field environments, a ruggedized handheld computer 302 maycommunicate with tribometer. Base station 302 also may be a notebook, ora laptop, or another computing device capable of bidirectionalcommunication with tribometer 300, and having an operator interface toenter data, to perceive information, or both. A wired link can be a USBconnection, a IEEE Std. 1394 connection, an RS-232 connection, an RS-422connection, an unshielded twisted pair (UTP) connection, or aController-area network (CAN) connection. A wireless link can be a USBOn-the-Go™ wireless link, an IEEE Std. 802.11a/b/g/n link, an IEEE Std.802.15 link (personal area network), or any other suitablenon-interfering ISM-type wireless link. Base station 302 can be used toset a value of at least preselected parameter used to control operationof tribometer 300, for example, rate of controllable propulsion forceapplication, sampling rate for sensed force values from force sensor310, or the number or type of sensed force, speed, temperature,humidity, or other sensed data collected from tribometer 300.

Base station 302 is used to initiate self-propulsion, for example, bytransmitting a “BEGIN TEST” signal to tribometer 300, or by otherwell-known signal techniques, using an established communication link,thereby removing idiosyncratic, irreproducible variations in forcemagnitude and direction, applied by a human test device operator to atest material relative to a test surface. Tribometer 300 is configuredto produce results that are both accurate and reproducible from test tocomparable test, thereby allowing substantially direct measurement offrictional forces between a preselected test material and a preselectedtest surface, and computation of μ_(s). Consequently, the reckoning ofSCOF, in accordance with the application of the present invention,produces SCOF values grounded in actual values of static friction, andnot derived or confounded by kinetic processes. Approximately 20 toabout 30 automatic force readings may be taken, stored in memory, anddisplayed during each test epoch, although more or fewer samples may betaken.

In alternative embodiments, drive unit 305 is configured to be astationary base 305, in which navigation unit 345 or steering unit 327may not be used. Also, controllable motive unit 320 may be configured todraw a cord (not shown), such as by a pulley urged by force engine 325.Force sensor 310 also may be configured to sense force delivered toadjustable test sled 315 by controllable motive unit 320, by forceengine 325, or by both unit 320 and engine 325. Test sled 315 can beconfigured as a weighted box with up to four (4) to six (6) facets, orsides, with a facet being configured to receive a predetermined materialsample. A predetermined weight, such as 5 lbm can be a weight caissonsecured in a closed box of test sled 315 by an epoxy resin adhesive, toprevent weight movement. Thus, alternative embodiments of anintrinsically calibrated tribometer 300 may include those that areconfigured with a stationary base 305, as well as those which arenon-stationary, for example the aforementioned self-propelled drive unit305. In the alternatives, test sled 315 receives an applied tractiveforce from which static or dynamic friction forces and correspondingstatic or dynamic coefficients of friction may be measured. Furthermore,in the alternatives, a weighted shoe may be used in place of adjustablecontact sled 315, so that frictional parameters of an actual shoe may bemeasured. Unlike some prior tribometers, the test piece, e.g. contactsled, test sled, or shoe 315, is not slammed against the test surface,inducing error by creating a vacuum between the test piece and the testsurface, and by oscillations which may be introduced into a forcemeasurement.

Tests, self-calibrations and measurements performed by the embodimentsof intrinsically calibrated tribometer 300 can be reproducible andoperator independent, unlike the current state of the tribometer art.Moreover, tribometer 300 can be intrinsically calibrated using a knowntest object 315, which may be, without limitation, a contact sled, atest sled, or another test object such as a shoe. Intrinsic calibrationcan be performed by providing a gantry rising upwards over tribometerunit 305 and having a pulley at the apex. Known test object 315 may beattached to force sensor 310 using a strong, substantially non-elasticcord and force engine 325 may apply tractive force, causing known testobject to be suspended off of the base resting surface. Once suspended,measurements of known test object weight 315 may be taken, and used tointrinsically calibrate tribometer 300. The intrinsic calibration isoperator-independent. Intrinsic calibration using a test object can leadto reliable, reproducible, and verifiable determination of coefficientof friction for each test epoch or period. This intrinsic calibrationconfiguration and technique also are not known in the current tribometerart.

The embodiment of FIG. 4, depicts a cutaway and extended version ofself-propelled tribometer 400, including drive unit 405. Tribometer 400is similar to tribometer 200 and tribometer 300, with like parts havingsimilar reference numbers. Tribometer 400 also is configured to performoperator-independent self-calibration, greatly reducing or eliminatingoperator-induced error. Contact sled 415 is depicted as having carrierframe 402 configured to support a six-sided faceted sample carrier 463.Each facet 465, 475 has a workpiece carrier 467, 477, which isconfigured to releasably retain a respective preselected material sample468, 478. In general, each facet 465, 475 may have a length greater thanits width, and each facet 465, 475 can be aligned with a longitudinal(X) axis of frame 402. Sample carrier 463 may include a centralprotuberance 444, or knob, to facilitate rotation of sample carrier 463on frame 415. Such rotation brings preselected material sample 468, 478into contact with preselected test surface 450. Lock 455 may be providedon sample carrier 463 to prevent inadvertent rotation or misalignment ofsample 468, 478 with surface 450. Lock 455 may have locking elements onboth frame 402 and carrier sled 415, although other configurations arecontemplated. Lock 455 also may provide registration of sample carrierposition and, by extension, correspondence of a preselected samplematerial to a particular test data set. In some embodiments, lock 455 isconfigured to lock a preselected sample material under test to besubstantially parallel to test surface 450. In other embodiments, lock455 may be configured to permit a selectable degree of off-parallelalignment, where such alignment would be useful in selecting from amonga range of possible interfacial test areas between the sample under testand the test surface.

Weight caisson 460 is depicted as being disposed, in whole, in contactsled 415 solely to facilitate exposition. Weight caisson 460 may bedisposed in part, or in whole, on one or both of drive unit 405 andcontact sled 415. Also, weight caisson 460 is illustrated as being acontainer having lateral elements and a fixed-weight caisson; however,weight caisson 460 also may be configurable to vary the gravitationalforce holding preselected material sample 468, 478, for example, insample carrier 467 or 477 in contact with preselected test surface 450.

Power unit 490 can provide power to one or more elements of drive unit305, for example, force engine 425 and controller 430. Power unit 490may be a battery, a fuel cell, or other energy source configured tosupply the power of tribometer 400. In certain selected embodiments,power unit 490 may receive at least some power through a lightweightwired electrical link, or a wireless link, such as an electromagneticinduction power transfer source.

FIG. 5 depicts a view of tribometer 400, illustrating an end of ahexagonally-arranged faceted sample carrier 463. Weight caisson 460 isdepicted amid the hexagonal sample carrier, upon which facets, such asfacet 475, are disposed sample carriers, such as carrier 477, which inturn, retains test sample material 478. Typically, test sample material478 is held in place on surface 450 while a tractive force is providedby drive unit 405.

FIG. 6 illustrates a top view of a contact sled 415 in FIG. 4, whichincludes carrier frame, and force sensor 410. Again, a hexagonal-facetedversion of contact sled 415 is shown, although this configuration is notessential to all embodiments. Weight caisson 460 can be seen asincluding turning knob 444, which can allow a user to turn samplecarrier 463 to a selected test sample, such as sample 468 to be place incontact with test surface 450 (of course, sample 468 would be rotated tobe disposed at least partly in contact with test surface 450).

FIG. 7 is a side view of a stationary embodiment of intrinsicallycalibrated tribometer 700, in which base 705 can be configured to remainsubstantially stationary and to apply tractive force 703 to test sled715 on test surface 750. Tribometer 700 can be similar to tribometer300, with similar numbers having like meaning. Base 705 generally can beoutfitted and configured to include functional units described in FIG.3, with respect to intrinsically calibrated tribometer 300, without useof self-propulsive or navigational elements. However, positionalinstrumentation (e.g., GPS or other GIS sensor) may be used,particularly when tribometer 700 is configured for use with pluraltribometers. Nevertheless, the operator-independent intrinsicallyself-calibrating capabilities, and other aforementioned force andcoefficient of friction mensuration features, also are included instationary embodiments. Tribometer 700 may communicate usingcommunications unit 740, as well as by keyboard 792, by display 794, orby both. Tribometer 700 may be coupled to test sled 715 by way of hook711 and cord 709. Force engine 725 can apply motive force tocontrollable motive unit 720, which may be a spindle, drum, pulley orother uptake apparatus, and frictional force 701 may be measured byforce meter 710, as well as by sensing a state of force engine 725(e.g., voltage, current, power, or torque).

Test sled 715 includes a multiple carrier test bed 763, which may beconfigured to hold at least one carrier 766 for a test specimen 768,which may be placed in contact with test surface 750. Test sled 715 mayoperate similarly to contact sled 415, in that test sled 715 generallymay have tractive force, F_(T), 703 applied by, and be dragged towards,base 705. Test specimen 768 can be any type of material about whichfriction characteristics may be of interest. One such material includesshoe sole material; another may include shoe heel material, althoughtest specimen and test surfaces are not constrained. Indeed, manysurface samples, which may be standardized surface samples, mayinterchangeably be placed into carrier 766 as a test specimen 768.Typically, tractive force, F_(T), 703 is opposed by static friction,f_(S), or by dynamic friction, f_(K), depending respectively on thekinetic state of the tribometer 700. In general operation, a force 701resistive to the applied tractive force 703 exists while test sled 715remains at rest on test surface 750, and can be representative of staticfriction; on the other hand a force 740 resistive to the appliedtractive force exists while test sled 715 is in motion, and can berepresentative of dynamic friction. Typically, as tractive force 703 isapplied to resting test sled 715 on test surface 750, the magnitude ofmeasured static friction 701 increases (similar to the force profile inFIG. 1B), until an inflection point is reached, at which motion occursin test sled 715. During operation, tractive force may be measuredduring stationary and moving periods, for example, about 10-20 forcemeasurements, and also may be averaged. Tractive force 703 can berepresentative of the opposing frictional force. The tractive force canbe representative of a measured dynamic coefficient of friction 740,when moving or of a measured static coefficient of friction whentractive force has not yet induced motion in test sled 715. Test sled715 can be weighted to achieve actual weight “W,” and force engine(e.g., motor) 725 in body 705 can pull cord 709 coupled to test sled715. Controller 730 in base 705 can be a computer, which can cause forcemeasurements “F” to be taken on pulled cord 709, such that the ratio“F/W” may be calculated automatically. Computer 777 can be external butcoupled to controller 730. Conveniently, this method can producecalibration values as well as test values. Because of a concern aboutthe impact of “stiction” forces, when motion is incipient, a first valueof dynamic friction (or last value of static friction) representative ofstiction may be discarded. Similarly, a first averaged value may bediscarded to eliminate “stiction” from analysis. However, all values maybe retained, if so configured. Computer 702 may be configured to presenton a display, measured values and other indicia of operation. Suchindicia may include, without limitation, time, date, ambienttemperature, atmospheric pressure, ambient light levels, or ambienthumidity. Force measurements may be proportional to a voltage used bymotor 707 used to create a force to pull cord 709.

In this way, device 700 may be configured to measure both static anddynamic friction. Base 705 can be weighted to remain stationary, or,alternatively, it can be configured to clamp or to adhere to a surfaceproximate to test surface 750. With these principles in mind, test sled715 can be outfitted with at least one test specimen 768, which is heldin contact with test surface 750. The measured force interactionsbetween test surface 750 and test specimen 768, at rest and duringmovement, can be characterized by measured static friction and measureddynamic friction, respectively. In some embodiments, test sled 715 maybe weighted, for example, with a five pound weight, for example, inweight caisson 760, although other weights may be used. In addition,test sled may be configured to be outfitted with different types of testspecimen 768, for example, to test different types of specimen, withoutlimitation, a shoe material, a non-skid surface, or an unquantifiedspecimen to be analyzed. Tribometer 700 may be intrinsically calibrated,as described relative to FIG. 9.

FIG. 8 exemplifies a test period 800 having four (4) test epochs801-804. The illustrated values of each epoch 801-804 are similar to thevalues shown in FIG. 1B. However, it is shown that force 810 falls to aminimum 805-807 between epochs 801-804, indicating that the frictionmensuration device is stopped and at rest. As tractive force is applied820-824, the nonce (roughly, the peak value) 850-853 of each test periodis indicative of incipient motion in the test piece. Comparable valuesobtained during each test epoch can be averaged and be subjected toother statistical analyses. Each test epoch 801-804 may be about 5 to 8seconds in length, and a typical test period 800 may include betweenabout 5 and about 10 epochs, such that a test period 800 may be betweenabout 25 sec. to about 80 sec., a much shorter time than current testingconsumes. Longer or shorter test epochs 801-804 or periods 800 may beused but, for many instances between about 5 to about 10 test epochs801-804 can be sufficient, producing accurate, reliable, andreproducible statistical values.

Turning to FIG. 9, stationary intrinsically calibrated tribometer 900,which can be similar to tribometers 300 and 700, can include base 905,which may be configured to substitute in place of test sled 915, a testarticle 999, for example, a shoe or other article of study. In the caseof a shoe, base 905 may be attach to the test subject shoe 999 by asimple hook 911 and inelastic cord 909 arrangement. Tractive forces thenmay be applied by base 905 to the test subject shoe 999, as shoe restson test surface 950. As before, the transition from at-rest to in-motionsignifies the transition from static friction to dynamic friction. Wherethe circumstances of a fall mishap are being investigated forensically,for example, the configuration of base 905 may permit the static anddynamic friction forces of an actual shoe of a fall mishap victim to betested against the actual surface upon which the victim slipped andfell. Test article 999 may be weighted, for example, using test weight960, to simulate the victim's weight upon a surface 950 at the moment ofsimulated fall. Force and weight measurements made by computer 902 maybe used to determine static coefficient of friction, dynamic coefficientof friction, or both, of test surface 950, test specimen 999, or both.Controller 930 can operate display 994 to display test indicia and tocreate a hard-copy (printout) of stored, measured, or calculated data.Controller 930 may be similar to the functional units of FIG. 3, andinclude flash memory in which to store indicia or data, as well as anUSB port to facilitate transfer of data or indicia to, for example, aUSB-configured portable memory device. Display 994 may be cooperatedwith a hard-copy printer (not shown), for example, using a WLAN or WPANprotocol, and may display the actual ramp-up of applied static force,and transition to dynamic force application.

In a prototype device in accordance with the aforementioned principles,eight (8) D-size battery cells 990 may be used by base 905 to providebase weight, to power force engine 925, and to operate computer 930.Instead of a spindle or pulley-type device, force engine (motor) 925 maypower a screw rod 920 coupled to force back and forth to achieve thedesired force application. The screw rod 920 may have a mechanicalauto-stop to prevent over driving. Similarly, force engine 925 caninclude an electrical, mechanical, or electromechanical stop to preventover-driving. Also, cord 909 may be threaded through pulley 982 at theapex of stanchion 980, to use controllable motive unit 920 to performfriction force measurements as well as weight measurements. It has beendetermined that the 8 D-cell batteries may be capable of providing up toone year of intermittent service of device 900, and of providingsubstantial stabilizing weight to base 905. Base 905 (and base 705 intribometer 700) may be made of an aluminum block, include an offset pairof wheels 963, and a retractable handle 965 to facilitate movement intransit much like a piece of rolling (pull-along) luggage. Theretractable handle may be fitted with a removably attachable shelf 967to permit handy access to a keyboard, and/or a display and, perhaps, aprinter. Intrinsically calibrated tribometer 900 can include one or morehandles and securable elements (not shown) by which to reversiblyconvert device 900 into a single physical unit, convenient for transportand storage. In addition, base 905 may include a stanchion 980 havingpulley 982, from which test sled 715 or test device (e.g., shoe) 999 maybe suspended. Weight and force calibration also may be determined bylifting the suspended shoe 999 towards the apex of stanchion 980 viapulley 982 such that shoe 999 is free from contact with surface 950 andis suspended from pulley 982. Weight readings may be made and recorded,so that calibration can reflect the actual weight of the shoe or othertest piece.

In view of the foregoing, methods for friction measurement are provided,one method including applying an increasing tractive force to a testsled at rest at the beginning of a test epoch or a test period;measuring the tractive force on the test sled at rest on a test surfaceduring a test epoch or a test period; identifying the maximum tractiveforce applied at incipient motion of the test sled on the test surfaceduring a test epoch or a test period; identifying a nonce of one or moretest epochs signifying the transition between the test sled at rest andincipient motion; determining the static coefficient of friction of thetest sled relative to the test surface for a test epoch or a testperiod; measuring the tractive force used to continue motion in the testsled for a test epoch or a test period; stopping the tractive force atthe end of a test epoch or a test period; determine the dynamiccoefficient of the test sled relative to the test surface for a testepoch or a test period. The method also may include discarding ameasurement corresponding to a stiction measurement. The tractive forcemay be applied by driving a controllable motive element to coupled to aself-propelled device body; by coupling the self-propelled device bodyto a contact sled against a test surface; by increasing the tractiveforce until incipient motion of the contact sled occurs; by measuringthe tractive force against the contact sled applied at the nonce ofincipient motion; by measuring tractive force during contact sled motionduring a remainder of a test epoch; by determining the staticcoefficient of friction of the contact sled with regard to the testsurface; and by determining the dynamic coefficient of friction of thecontact sled with regard to the test surface.

Intrinsic calibration method may include applying tractive force to atest piece during a calibration period; suspending the test piece off ofa weight-bearing surface during the calibration period; and weighing thesuspended test piece.

The examples used herein are intended merely to facilitate anunderstanding of ways in which the invention may be practiced and tofurther enable those of skill in the art to practice the embodiments ofthe invention. Accordingly, the examples and embodiments herein shouldnot be construed as limiting the scope of the invention, which isdefined solely by the appended claims and applicable law. Moreover, itis noted that like reference numerals represent similar parts throughoutthe several views of the drawings, although not every figure may repeateach and every feature that has been shown in another figure in order tonot obscure certain features or overwhelm the figure with repetitiveindicia. It is understood that the invention is not limited to thespecific methodology, devices, apparatuses, materials, applications,etc., described herein, as these may vary. It is also to be understoodthat the terminology used herein is used for the purpose of describingparticular embodiments only, and is not intended to limit the scope ofthe invention.

What is claimed is:
 1. A friction mensuration device, comprising: aforce engine configured to apply a tractive force to a test piecedisposed on a test surface; a controller coupled to the force engine,and imposing a tractive signal to the force engine, a sensed forcesignal to the controller selectively actuating the force engine toproduce the tractive force; and a force sensor coupled between the forceengine and the test piece, the force sensor producing the sensed forcesignal responsive to a sensed force between the force engine and thetest piece, wherein the force engine is intrinsically calibrated to theNormal force, wherein the test piece receives the tractive force,wherein the test piece translates the tractive force into a test forceapplied to the preselected material sample coupled to the test piece,wherein the test force applied to the preselected material sample isrepresentative of a friction force between the preselected materialsample and the test surface, wherein one of the preselected materialsamples comprises a control sample including a piece of footwear or aresin rubber or a composite material, wherein a sensed force measurementrepresentative of a stiction force is rejected, and wherein ameasurement by the device is reliable, reproducible, verifiable, andoperator-independent.
 2. The friction mensuration device of claim 1,wherein the device further comprises a drive unit coupled to the forceengine and having a stationary drive unit coupled to the test piece. 3.The friction mensuration device of claim 1, wherein the device furthercomprises a drive unit coupled to the force engine and having aself-propelled drive unit coupled to the test piece.
 4. The frictionmensuration device of claim 1, wherein the sensed force is a staticfriction force.
 5. The friction mensuration device of claim 1, whereinthe sensed force is a dynamic friction force.
 6. The frictionmensuration device of claim 1, wherein, in response to a receivedsignal, the controller actuates the force engine to impose a frictionforce between the test surface and the preselected sample material. 7.The friction mensuration device of claim 1, further comprising: amensuration memory coupled to the controller and having memory storageunits, the mensuration memory being configured to receive from thecontroller a plurality of representations of the sensed force, and themensuration memory configured to retain respective ones of the pluralityof representations of the sensed force.
 8. The friction mensurationdevice of claim 1, wherein the test piece further comprising: a contactsled including a carrier having facets, wherein each of the facetsincludes a sample carrier thereon, each sample carrier being disposed toreleasably retain a respective preselected material sample, and whereinthe carrier can be disposed to position one of the preselected testmaterials in contact with the surface.
 9. The friction mensurationdevice of claim 1, further comprising: the force engine applies thetractive force during a first portion of a test epoch, corresponding tothe static coefficient of friction.
 10. The friction mensuration deviceof claim 1, wherein one of the preselected sample materials is a controlsample material.
 11. The friction mensuration device of claim 11,wherein the control sample material is a piece of footwear.
 12. Thefriction mensuration device of claim 11, wherein the control samplematerial is a resin rubber constituent of a preselected rubber solingmaterial.